1. Field of the Invention
This invention relates generally to gas discharge devices, and more particularly to a ground fault detector for power supplies used with cold cathode tubes.
2. Discussion of the Related Art
Cold cathode tubes, also known as neon tubes or gas discharge devices, use an ionization process to provide light. As depicted in FIG. 1, a cold cathode tube 12 is typically a vacuum-sealed glass tube 14 that is filled with inert gas 16 such as argon or neon. Tube 14 is fitted at each end with a metal electrode 18A and 18B to provide an electrical contact with inert gas 16. Pumping outlet 20 allows inert gas 16 to be sealed in tube 14.
The tubes may be fabricated in many shapes. Diameters of 6 mm to 18 mm are typical. As shown in FIG. 2, a tube may also be formed by a cavity 24 formed inside a glass material such as glass plate 22, again with electrodes 18A and 18B and outlet 20.
In operation, electrodes 18A and 18B are connected to a high voltage source. When connected to a high voltage source, the ionization process is initiated in which the atoms of the inert gas 16 are stimulated, and the tube will then glow with light from the energy spectrum that depends upon the gas type. For example, a neon tube will glow ruby red, mercury vapor will glow blue-green, and argon will glow pale blue.
Once ionized, a constant current is maintained through the inert gas at a voltage referred to as a running voltage. This constant current is typically in the range of 10 to 120 mA. In order to ionize the gas initially, a striking voltage of approximately 1.5 times the running voltage is provided to the electrodes.
The striking and running voltages are typically directly proportional to the tube length, and are typically in the range of 500-8000 Vrms for tubes having a length of approximately one foot to a length of several feet. The luminous intensity of inert gas 16 is directly proportional to the current that flows through inert gas 16.
An example of a known power supply for cold cathode tubes is depicted in FIG. 3. A high voltage converter 30 receives an input voltage V.sub.in, and generates an output voltage V.sub.out. V.sub.in may be an AC supply such as 110 VAC or 220 VAC for household or commercial applications or a DC supply such as 12V for automotive applications. The output voltage V.sub.out is provided to cold cathode tube 12 by high voltage cables 32. Typically, V.sub.out has a square shape waveform, and represents an open circuit voltage in the range of 1,000V to 15,000V. Due to the potential safety hazards, it is desirable that the high voltage cables 32 be as short as possible. This consideration limits the size and weight of the high voltage converter 30, since in order to keep the high voltage cables 32 short, the high voltage converter 30 is typically mounted as close as possible to cathode tube 12. Also for safety reasons, high voltage cables 32 are often installed in a conduit. The installation of the high voltage cables requires special consideration and must often adhere to strict safety codes. Furthermore, if the high voltage wires must pass through a wall, then a special insulator is required for passing the wires through the wall in order to comply with safety requirements.
Another characteristic of cold cathode tubes is that inert gas 16 within cathode tube 12 creates an apparent negative resistance. As inert gas 16 ionizes, the resistance as sensed by high voltage converter 30 decreases, causing the current within the tube to rapidly increase when power is initially applied to cathode tube 12. This rapid increase of current will cause instability in a tuned circuit within high voltage converter 30, which is providing power to cathode tube 12. The rapid current increase may in some instances damage the tuned circuit if implemented as a solid state power oscillator. Therefore, it has been necessary to provide a current limiting inductor in series with cathode tube 12, between the tube 12 and high voltage converter 30, in order to regulate the load current. However, such an arrangement causes RFI (Radio Frequency Interference) and EMI (Electromagnetic Interference) difficulties, because of the resulting unbalance between the load current limiting inductor and the tuned circuit providing the high voltage. Due to the harmonics that would be generated otherwise, the tuned circuit must be physically located close to the load current limiting inductor; this limits the options where to physically arrange high voltage converter 30.
In conventional circuits that include cold cathode tubes, current transformers are often used to sense a current imbalance caused by a ground leakage current returning to a source through an unintended ground circuit path. A protection device may isolate the circuit including the cold cathode tube from a power supply when a fault in the circuit is detected.
FIG. 4 shows a conventional circuit for sensing ground fault current. A bridge rectifier 40 connects an AC line voltage to an inverter 42 through inputs 43. Inverter 42 is connected to primary ground 45, which may be zero volts. Outputs 44 of inverter 42 drive the primary side of a transformer T1. Cathode tube L is connected to the secondary side of transformer T1 through outputs 51 and 52 of transformer T1. Midpoint M of transformer T1 is connected to secondary ground 53, for example, earth ground, through the primary side (P) of current transformer CT. The secondary side (S) of current transformer CT is connected to the shunt resistor R.sub.s1, which in turn is connected at one end to primary ground 45 and to an input 50 of comparator 48. Comparator 48 also receives threshold voltage V.sub.th. An output 49 of comparator 48 is connected to input 46 of inverter 42. In addition, a ground fault condition in the circuit may be represented by, for example, R.sub.F1 and R.sub.F2, which are shown connected to secondary ground 53. A ground fault current which develops may flow back into the secondary side of transformer T1 through outputs 51 and 52, and may flow through midpoint M. Any ground fault current flowing through midpoint M may also leak back into the primary side P1 of transformer T1 through primary ground 45 connected to current transformer CT.
A method of operating the circuit described in FIG. 4 includes connecting the primary side (P) of the current transformer CT to a midpoint M of high voltage transformer T1. Current through the secondary side (S1 and S2) of transformer T1 is sensed by current transformer CT through the connection of the high voltage transformer midpoint M to secondary ground 53. Therefore, any ground fault current flowing in the circuit, as represented by R.sub.F1 or R.sub.F2, flows from midpoint M through the primary side of the current transformer to secondary ground 53. A secondary current is dependent on the turns ratio of the current transformer CT. For example, for a 1 to 1 ratio of the transformer CT, substantially the same amount of fault current flows into a resistor R.sub.s1, connected across a secondary side of the transformer CT, as the fault current flowing through the primary side of current transformer CT. A voltage across this shunt resistor R.sub.s1 is measured and sent to comparator 48 to detect a ground fault condition. Comparator 48 compares the voltage V.sub.s1 across R.sub.s1 to a threshold voltage V.sub.th. For example, when the V.sub.s1 voltage exceeds threshold voltage V.sub.th, due to a ground fault, the output of comparator 48 is used to send a signal to inverter 42 to inhibit the operation of inverter 42.
This technique has several drawbacks. For instance, the current transformer CT is relatively large in order to pass a frequency in the range of 20-30 kHz of the high voltage inverter with a minimum amount of loss. The relatively large current transformer is needed to achieve low loss at the inverter fundamental frequency since the coil impedance is in parallel with the shunt resistor R.sub.s1 and causes an error. Furthermore, the magnetizing current of the core produces an additional error. To minimize these errors the core cross section and the number of turns must be made large enough as to cause a small error of the measured current. The large transformer adds size and often adds cost to the above-described fault detector. In addition, the parameters of the transformer CT must be carefully taken into account in order to set an accurate threshold voltage V.sub.th, since any loss in the transformer CT results in an inaccurate detection of ground fault current.